I. Field of the Invention
The present invention relates generally to electronic circuits and, more particularly, to power-dividing and/or power-combining circuits.
II. Description of Related Art
Millimeter wave communication, radar, electronic warfare systems, also known as microwave systems, conventionally use amplifiers to increase the power output signal level. An increase in the output power for the microwave system achieves many advantages, e.g. improved operating range, improved communications, and the like.
In order to amplify the microwave transmission signal, conventionally the signal is divided by a splitter having one input and two outputs so that one output signal is coupled as an input signal to one amplifier while the other output signal is connected as an input signal to the other amplifier. The outputs from the amplifiers are then connected to a combiner so that the output signal from the combiner is equal to the input signal amplified by the two amplifiers minus any losses in the splitter or combiner.
Furthermore, the splitter, also known as a divider, is identical to a combiner, but used in the reverse direction. For example, a splitter or divider would have one input and two outputs while the combiner has two inputs and one output so that the divider component is a mirror image of the combiner component. For that reason, the term “combiner/divider” is used interchangeably since a divider may be used either as a divider or a combiner depending upon the fashion in which it is connected to the amplifier circuit.
In order to achieve efficient power amplifier designs for microwave communications, it is critical that the path length from the outputs of the splitter and to the inputs of the combiner be equal. Otherwise, the signals from the amplifiers do not add constructively and power is lost. Likewise, it is necessary that the phase shift, if any, from the outputs of the splitter and to the inputs of the combiner be the same for each amplifier in order for the power output signals from the amplifiers to add or combine constructively.
One inherent problem with power signal level amplification in microwave systems is that the power amplifiers located between the divider and the combiner each exhibits an input reflection coefficient so that a portion of the signal provided by the divider to the power amplifier is reflected back to the input of the power amplifier circuit. This reflected signal effectively reduces the overall power input to the power amplifiers. This, in turn, reduces the overall efficiency of the power amplifier.
In order to reduce the adverse effects of the reflected power from the power amplifiers, there have been previously known balanced microwave power amplifier circuits which eliminated, or at least greatly reduced, the adverse effect of reflected power from the power amplifiers. These previously known balanced amplifier circuits included a first phase delay of 90° in between the divider output and the first amplifier while the output from the first amplifier was connected as an input signal to the combiner. Conversely, the input of a second amplifier was connected directly to a second output from the divider while the output from the second amplifier was electrically connected through a 90° phase shift delay line to a second input of the combiner. In operation, and assuming that both amplifiers are identical in operation to each other, a reflected signal from the first amplifier exhibits a phase shift of 180° at the output from the divider since the signal ultimately resulting in the reflected signal passes through the 90° delay line twice. Conversely, the phase of the reflection from the second amplifier back to the divider exhibits 0° phase shift, relative to the one reflected from the first amplifier. Since the magnitudes of the reflected signals from both amplifiers are equal, the reflected signals from the two amplifiers cancel each other.
These previously known balanced combiner microwave power amplifier circuits, however, have been limited to a two amplifier configuration between the divider and the combiner. In many situations, however, a simple two amplifier circuit produces insufficient output power for the microwave system.
In some circumstances, it may be possible to cascade two or more balanced power amplifier circuits together to increase the overall power output of the microwave system. However, the addition of extra divider/combiner pairs, each having a pair of power amplifiers electrically connected there between, increases the overall size and bulk of the microwave power system. In many situations, such as monolithic microwave/millimeter wave integrated circuits (MMICs), the cascading of multiple balanced pairs of amplifiers is simply impractical due to the limited space available on the MMIC substrate.